Modern microwave transmitter generally require an accurate control of the radio frequency (RF) transmit power. In the wireless applications, automatic power level control, dynamic power control over various distances and accurate power level control to avoid excessive power to adjacent cells are a few examples of the importance of accurate power controls. FIG. 1A is an example of a conventional power detector application to achieve an accurate control of the transmitted power.
In addition to the accurate output power control in modern wireless transmitter, current advanced RF/microwave transmitters incorporate pre-distortion techniques or similar techniques to increase the output power, reduce system power consumption and increase power efficiency. Because of the low cost advantage and the implementation of digital signal processing, linearization of a power amplifier has become an important technology. Nearly all pre-distortion techniques require that a coupled RF signal at the output of the power amplifier be processed and corrected through digital or analog techniques. FIG. 1B illustrates an example of a conventional pre-distortion linearization application in current wireless system.
Further, an RF loopback is another important system requirement. The RF loopback is designed for system self-debug and calibration applications in current RF/microwave system. The RF loopback provides the system an internal RF path from the output of the transmitter to the local receiver input. With the feature of the RF loopback, the end-to-end test can be easily performed to test system calibration, or on-site system self-debug to minimize the cost related to product manufacturing, installation and field maintenance. FIG. 2A illustrates an example of a conventional RF loopback application in current wireless system.
Further, coherent power combining is another example of a system level RF coupler. To achieve maximum RF output power with the maximum efficiency, coherent power combining is used, and becomes one of the most efficient power combining methods. For example, in a phase RF power combining application, each transmitter has respective calibrated phase input signal, and each RF coupler of a transmitter is configured with a phase detector and adjusting feature. FIG. 2B illustrates an example of a conventional coherent power combing application in current wireless system.
To achieve some or all of the above advanced features, an RF transmitter needs to either have one RF coupler and split configuration as shown in FIG. 3A, or a dual RF coupler and split configuration as shown in FIG. 3B. In microwave and millimeter wave bands above 10 GHz, the output port is usually a waveguide due to its minimum transmission loss and optimum connection to the antenna. Microstrip is the most common used transmission technique due to easy manufacturing and low cost. FIG. 4 is an example of a compact microstrip to waveguide dual coupler transition, as described in the earlier patent application 61/673,161 “A Compact Low Loss Transition with an Integrated Coupler,” which is hereby incorporated by reference in its entirety.